Book reviews     

Nancy N. Rabalais  Gary R. Gaston  John W. Day  L. Harold Stevenson  James S. Latimer 《Estuaries and Coasts》1997,20(3):655-658

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Nutrient inputs to the Choptank River estuary: Implications for watershed management     

Lorie W. Staver  K. W. Staver  J. Court Stevenson 《Estuaries and Coasts》1996,19(2):342-358

Degraded water quality due to water column availability of nitrogen and phosphorus to algal species has been identified as the primary cause of the decline of submersed aquatic vegetation in Chesapeake Bay and its subestuaries. Determining the relative impacts of various nutrient delivery pathways on estuarine water quality is critical for developing effective strategies for reducing anthropogenic nutrient inputs to estuarine waters. This study investigated temporal and spatial patterns of nutrient inputs along an 80-km transect in the Choptank River, a coastal plain tributary and subestuary of Chesapeake Bay, from 1986 through 1991. The study period encompassed a wide range in freshwater discharge conditions that resulted in major changes in estuarine water quality. Watershed nitrogen loads to the Choptank River estuary are dominated by diffuse-source inputs, and are highly correlated to freshwater discharge volume. in years of below-average freshwater discharge, reduced nitrogen availability results in improved water quality throughout most of the Choptank River. Diffuse-source inputs are highly enriched in nitrogen relative to phosphorus, but point-source inputs of phosphorus from sewage treatment plants in the upper estuary reduce this imbalance, particularly during summer periods of low freshwater discharge. Diffuse-source nitrogen inputs result primarily from the discharge of groundwater contaminated by nitrate. Contamination is attributable to agricultural practices in the drainage basin where agricultural land use predominates. Groundwater discharge provides base flow to perennial streams in the upper regions of the watershed and seeps directly into tidal waters. Diffuse-source phosphorus inputs are highly episodic, occurring primarily via overland flow during storm events. Major reductions in diffuse-source nitrogen inputs under current landuse conditions will require modification of agricultural practices in the drainage basin to reduce entry rates of nitrate into shallow groundwater. Rates of subsurface nitrate delivery to tidal waters are generally lower from poorly-drained versus well-drained regions of the watershed, suggesting greater potential reductions of diffuse-source nitrogen loads per unit effort in the well-drained region of the watershed. Reductions in diffuse-source phosphorus loads will require long-term management of phosphorus levels in upper soil horizons. *** DIRECT SUPPORT *** A01BY074 00021  相似文献   

Physics and Chemistry of sulfur lakes on Io   总被引：1，自引：0，他引：1  

Jonathan I. Lunine  David J. Stevenson 《Icarus》1985,64(3):345-367

A model for a convecting sulfur lake, heated from below by a silicate magma chamber , is constructed and applied to major hot spot regions on Jupiter's satellite Io. We use a two-layer parametrized convection scheme for sulfur and silicates based on a local boundary layer analysis to calculate temperature profiles in the system and the maximum flux which can be extracted from the silicate magma in steady state. The results indicate that the highest-component temperature of some observed hot spots (J. S. Pearl and W. M. Sinton, 1982, In The Satellites of Jupiter (D. Morrison, Ed.), pp. 724–755. Univ. of Arizona Press, Tucson) is consistent with a convecting molten sulfur system, and the total flux from the most energetic spot, Loki Patera, is close to the maximum which can be extracted from molten silicates by convection. Simple hydrodynamic models of evaporative outflow from sulfur lakes indicate that the intermediate-component temperature of hot spots such as Loki can be identified with the evaporative sulfur flux which condenses in the atmosphere and over a wide area surrounding the lake(s). The ratio of warm to hot component fluxes for Loki and other hot spots is consistent with this interpretation, and evaporation sets a strong constraint on the maximum surface temperature for a steady-state lake. The Voyager IRIS continuum spectrum can be fitted by a sulfur lake model in which sulfur vapor condensing on the shore is assumed to radiate as a blackbody. The lifetime of such a lake, in steady state, based on evaporation and silicate cooling time scales is 1–100 years, implying long-term Earth-based observations could detect variations in the Loki thermal output. The model provides a useful interpretive tool for possible variability because it gives predictions for the relative thermal fluxes at different wavelengths. The sodium-sulfur phase diagram is also presented and used to show the evaporated lakes may leave behind a sodium-rich residue which could supply the torus with sodium. Finally, uncertainties in the model are assessed, including the lack of sulfur emission features in the Loki spectrum, and the alternative possibility that the SO₂ plume observed at Loki could be supplying the excess thermal flux.  相似文献   

The influence of trace amounts of water on the viscosity of rhyolites     

R. J. Stevenson  N. S. Bagdassarov  D. B. Dingwell  C. Romano 《Bulletin of Volcanology》1998,60(2):89-97

 As a major volatile in volcanic systems, water has a significant influence on the rheological properties of silicic magmas. This is especially so at minor water contents relevant to the emplacement of silicic lavas. To investigate the influence of water on the viscosity of natural rhyolitic obsidians, a novel strategy has been adopted employing parallel-plate and micropenetration techniques. Viscosities have been determined on three types of material: (a) raw water-bearing obsidians; (b) remelted (1650  °C, 1 atm) degassed glasses of the obsidians; and (c) hydrothermally hydrated (1300  °C, 3 kbar) obsidians. Ten natural rhyolitic obsidians (peraluminous, calc-alkaline and peralkaline) were employed: seven originated from lava flows and contained <0.2 wt.% H₂O, two samples were F-rich from pyroclastic successions, and one was an obsidian cobble with 1.5 wt.% water also associated with pyroclastic units. Melt compositions and water contents were stable during viscometry. The measured decreases in activation energies of viscous flow and viscosity with small amounts of water are much greater than the Shaw calculation scheme predicts. In addition, a marked non-linear decrease in η exists with increasing water content. In contrast to the case for peralkaline rhyolites, 0.1–0.2 wt.% water decreases activation energies significantly (up to 30%) for calc-alkaline compositions. These results have important implications for the ease of near-surface degassing of silicic magmas during emplacement and permit the testing of calculational models for viscosity, largely based on synthetic systems. Received: 7 July 1997 / Accepted: 6 April 1998  相似文献   

Influence of freshwater intrusion on microbial biomass in salt-marsh creeks     

R. T. Weiland  Thomas H. Chrzanowski  L. Harold Stevenson 《Estuaries and Coasts》1979,2(2):126-129

Adenosine triphosphate (ATP), particulate organic carbon (POC), pH, temperature, and salinity associated with the water column of several salt marsh creeks were monitored at 5 stations for 8 months. A gradient in mean salinity of 11.5‰ to 32.7‰ was observed in the creeks. No significant correlations (Pearson’s r) could be found among the variables measured at the station with the highest salinity. ATP and POC were found to be positively correlated at all other stations. Salinity was found to be negatively correlated with both ATP and POC only at a station with the second highest mean salinity (28.8‰) and could account for only 45.7‰ of the variation in ATP. The lack of significant correlations between salinity and ATP as well as the inability of salinity to account for a large portion of the variation in ATP suggested that salinity had little relationship to the level of total microbial mass.  相似文献   

A wind tunnel boundary-layer simulation of wind flow over complex terrain: Effect of terrain and model construction     

D. Neal  D. C. Stevenson  D. Lindley 《Boundary-Layer Meteorology》1981,21(3):271-293

The report presents the results of a wind-tunnel study of the flow of the natural wind over complex terrain. A 1:4000 undistorted scale model of Gebbies Pass in the South Island of New Zealand was prepared and tested in the boundary-layer wind tunnel in the Department of Mechanical Engineering at the University of Canterbury.Three forms of construction, viz., terraced, contoured and roughness-added, were compared. Velocity and turbulence profiles, Reynolds stresses and spectra were measured, and correlation of results between different types of construction was calculated. The terraced form was much simpler to construct but was found to be unsatisfactory. The correlation between the contoured and roughness-added models was as high as 0.94, although the roughness-added model made a significant difference to the results in the lower 20%; of the boundary layer. The results of these tests will be compared with results from the field in a future report.  相似文献   

Book reviews     

Robert S. Maples  Lovett E. Williams  L. Harold Stevenson 《Estuaries and Coasts》1992,15(3):428-428

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CENTRAL PLACE DEVELOPMENT IN A TIME-SPACE FRAMEWORK *     

Donald G. Janelle 《The Professional geographer》1968,20(1):5-10

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Book reviews     

Marie-José Gaillard  André F. Lotter  Norman D. Yan  Tony Stevenson  H. E. Wright 《Journal of Paleolimnology》1993,8(1):85-95

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Widespread inflation and drainage of a pāhoehoe flow field: the Nesjahraun, Tingvellir, Iceland     

John Alexander Stevenson  Neil Charles Mitchell  Michael Cassidy  Harry Pinkerton 《Bulletin of Volcanology》2012,74(1):15-31

This study describes the emplacement of the Nesjahraun, a basaltic lava flow that entered the lake Tingvallavatn, SW Iceland. High-resolution remotely sensed data were combined with fieldwork to map the flow field. Onshore, the Nesjahraun exhibits a variety of textures related to the widespread inflation and collapse of a pāhoehoe flow field. Its emplacement is interpreted as follows: Initially, the eruption produced sheet pāhoehoe. In the central part of the flow field, the lava has a platy-ridged surface, which is similar to some other lava flows in Iceland and on Mars. Here, the texture is interpreted to have formed by unsteady inflation of the brittle crust of stationary sheet pāhoehoe, causing it to break into separate plates. The ridges of broken pāhoehoe slabs formed as the plates of crust moved vertically past each other in a process similar to the formation of shatter rings. Upstream, fresh lava overflowed repeatedly from channels and tubes, covering the surface with shelly pāhoehoe. Formation of a 250-m-wide open channel through the flow field allowed the inflated central part of the flow to drain rapidly. This phase produced ‘a‘ā lava, which eroded the channel walls, carrying broken pāhoehoe slabs, lava balls and detaching large (>200 m long) rafts of compound shelly pāhoehoe lava. Much of this channelized lava flowed into the lake, leaving a network of drained channels and tubes in the upstream part of the flow. As in other locations, the platy-ridged texture is associated with a low underlying slope and high eruption rate. Here, its formation was possibly enhanced by lateral confinement, hindered entry into the lake and an elevated vent location. We suggest that formation of this type of platy-ridged lava, where the plates are smooth and the ridges are slabs of broken pāhoehoe, can occur without significant horizontal transport, as the surface crust is broken into plates in situ. This reconstruction of the emplacement of the Nesjahraun also demonstrates that high-resolution aerial survey data are extremely useful in the mapping and measurement of lithofacies distributions in large flow fields, but that fieldwork is still necessary to obtain the detailed textural information necessary to interpret them.  相似文献